With the recent rapid quick supply of electronic systems used in batteries for cellular phones, notebook computers, electric cars, etc., there is a rapidly increasing demand for small, lightweight and relatively higher-capacity secondary batteries. In particular, lithium secondary batteries have come into the spotlight as drive power sources for portable devices since they are lightweight, and have a high energy true density. Therefore, there have been extensive research and development efforts to improve the performance of lithium secondary batteries.
Lithium secondary batteries produce electric energy through an oxidation-reduction reaction when lithium ions are intercalated/deintercalated into/from negative electrodes and positive electrodes, both of which are made of an active material enabling intercalation and deintercalation of lithium ions, in a state in which an organic electrolyte solution or a polymer electrolyte solution is filled between the negative electrodes and the positive electrodes.
A transition metal compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), and the like is widely used as the positive electrode active material for lithium secondary batteries. Also, a crystalline carbon material having a high softening degree, such as natural graphite or artificial graphite, or a pseudo-graphite structure or amorphous carbon material, which is obtained by carbonizing a hydrocarbon or a polymer at a low temperature of 1,000 to 1,500° C., is generally used as the negative electrode active material. Since the crystalline carbon material has a high true density, it has advantages in that it is desirably used to pack an active material, and exhibits excellent potential flatness, initial capacity and charge/discharge reversibility.
Generally, soft carbon, hard carbon, or small-particle graphite has been used as the negative electrode active material to express the high-output performance of lithium secondary batteries. However, when such a carbonaceous negative electrode active material is used in the negative electrode, a discharge end portion of the negative electrode passes through a region to which the highest resistance is generally applied, that is, a discharge end portion of the positive electrode, resulting in an increase in resistance. As a result, the output of the secondary battery at a low state-of-charge (SOC) level may be remarkably reduced.